1. Field of the Disclosure
The disclosure relates generally to laser-based fabrication of subsurface objects in glass and, more particularly, to subsurface sight reticles.
2. Brief Description of Related Technology
Creation of a three-dimensional object inside a glass substrate (i.e. a sub-surface object) is becoming a common fabrication procedure. Systems have been developed to produce three-dimensional objects in souvenir “crystals” or like items. A number of companies offer custom-made souvenirs by using a three-dimensional photograph as the basis for subsurface formation of an object capable of producing an image of the photograph for a person looking at the crystal.
The objects in these inexpensive commercial items are typically created using nanosecond laser pulses. The resulting quality of the designs and images varies greatly between vendors, but in all cases images of the objects are visible due to the presence of relatively large laser-created scattering centers. Light, diffusing from the surroundings or from a punctual source, is deflected by the scattering centers into the eyes of the person viewing the object. The image appears to be brighter than the rest of the glass block, because the unaltered glass surrounding the object does not scatter light towards the viewer.
Other subsurface objects have been created through the use of a mixed composition glass that changes its color when exposed to proper laser illumination. Some glasses form such “color centers” when exposed to the proper excitation. These color centers can block incoming visible light. This phenomenon has been used to create subsurface objects in a block of mixed glass. Unlike the previously described approach, images of these objects are visible because the incoming light is absorbed by the color center (rather than scattered). Unfortunately, color centers generally have a limited lifetime, especially when exposed to higher temperatures. While special glass treatments or glass compositions can reduce this short lifetime drawback, fused silica or fused quartz does not form significantly visible color centers.
Ultrashort laser pulses have also been used to create smaller scattering centers (relative to those obtained with nanosecond pulses) and, thus, create finer imagery. However, these smaller scattering centers are not as efficient at scattering light. As a result, ultrashort pulses are typically not used for artistic applications. Instead, ultrashort pulses are commonly used when it is desirable to generate a weak or concealed marking when, for example, discreetly marking an item can help to prevent counterfeit reproductions.